Optical System with Optical Parametric Oscillator

ABSTRACT

An optical system and method that use a pump laser, an optical parametric oscillator, and two second harmonic generators to generate three colors of laser light. A recirculating optical sub-system includes a gain-guided optical parametric oscillator and one of the second harmonic generators and has four lenses that form two collimated optical beams between the optical parametric oscillator and the second harmonic generator.

BACKGROUND OF THE INVENTION

Optical systems with optical parametric oscillators (OPOs) may be used to generate multiple wavelengths of laser light from one pump laser beam. In the OPO, parametric amplification in a nonlinear crystal converts the pump laser wavelength into two more wavelengths of light, so an optical system with one pump laser and one OPO may produce at total of three wavelengths of visible light which are useful for applications such as full-color digital image projection.

SUMMARY OF THE INVENTION

In general, in one aspect, an optical system including an optical parametric oscillator (OPO), a second-harmonic generator (SHG), a first lens which passes light between the OPO and the SHG, a second lens which passes light between the OPO and the SHG, and a third lens which passes light between the OPO and the SHG.

Implementations may include one or more of the following features. A fourth lens may pass light between the OPO and the SHG. The first lens may pass a collimated beam segment to the second lens and the third lens may pass a collimated beam segment to the fourth lens. The first lens, the second lens, the third lens, and the fourth lens may be arranged to form beam segments with a non-rectilinear shape. There may be a recirculating optical subsystem which focuses the beam waist in the OPO to the same location for 10 circuits around the recirculating optical subsystem with a variation of less than 10% of the width of the beam waist. There may also be a first short wave pass (SWP) filter, a second SWP filter, a minor, and a third SWP filter arranged to form a recirculating optical subsystem. The first SWP filter and the second SWP filter may be mounted in an optical module with the OPO and the minor and the third optical SWP filter may be mounted in an optical module with the SHG. The optical system may generate two infrared beams. Each infrared beam may be frequency doubled to make a visible beam. One visible beam may be in the range of 430 nm to 480 nm and the other in the range of 600 nm to 680 nm. One visible beam may be at 452 nm and the other at 621 nm. The OPO may be pumped by a beam in the range of 510 nm to 550 nm. The pump beam may be 523.5 nm or 540 nm.

In general, in one aspect, an optical system including a first SWP filter, an OPO, a second SWP filter, a first lens, a second lens, a minor, an SHG, a third SWP filter, a third lens, and a fourth lens. The OPO passes light from the first SWP filter to the second SWP filter, the second SWP filter passes light from the OPO to the first lens, the first lens passes light from the second SWP filter to the second lens, the second lens passes light from the first lens to the minor, the minor passes light from the second lens to the SHG, the SHG passes light from the mirror to the third SWP, the third SWP passes light from the SHG to the third lens, the third lens passes light from the third SWP to the fourth lens, the fourth lens passes light from the third lens to the first SWP filter, and the first SWP filter passes light from the fourth lens to the OPO.

In general, in one aspect, an method of generating light including the steps of focusing a pump beam into an OPO, forming an idler beam in the OPO, focusing the idler beam through a first lens and a second lens into a first SHG, and recirculating the idler beam through a third lens and a fourth lens back into the OPO.

Implementations may include one or more of the following features. The beam segment between the first lens and the second lens may be collimated and the beam segment between the third lens and the fourth lens may be collimated. Additional steps may include forming a signal beam in the OPO, separating the idler beam from the signal beam and the pump beam, forming a first second-harmonic beam from the idler beam in the first SHG, separating and outputting the first second-harmonic beam, separating and outputting the remaining pump beam, focusing the signal beam into a second SHG, forming a second second-harmonic beam in the second SHG, separating and outputting the second second-harmonic beam, and dumping the remaining signal beam.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING

FIG. 1 is a schematic view of an optical system with an optical parametric oscillator;

FIG. 2 is a schematic view of a recirculating optical subsystem with four relay lenses;

FIG. 3 is a schematic view of a recirculating optical subsystem with four relay lenses showing additional details of the optical beams;

FIG. 4 is a schematic view of a non-rectilinear recirculating optical subsystem; and

FIG. 5 is a flowchart of a method of generating light.

DETAILED DESCRIPTION

A detailed description of an optical system generating three colors of visible light using an optical parametric oscillator may be found in U.S. Pat. No. 5,740,190, the complete disclosure of which is incorporated herein by reference. Starting with a visible pump beam, an OPO makes an infrared signal beam and an infrared idler beam. By choosing a specific temperature and other characteristics of the OPO crystal, the signal beam and idler beam may be tuned as desired. For example, if the pump beam has a wavelength of 523.5 nm, the two additional beams created may be at 898 nm and 1252 nm which are in the infrared region. By using two second-harmonic generators, the 898 nm light can be frequency doubled to 449 nm light which is blue, and the 1252 nm light can be frequency doubled to 626 nm which is red. This produces red, green, and blue colors of visible light that may be used by a digital image projection system. By choosing a different temperature of the OPO, the infrared wavelengths may be 904 nm and 1242 nm which can be frequency doubled to visible wavelengths of 452 nm and 621 nm which fit the color gamut specified for digital cinema. Other wavelengths within the blue or red range may give acceptable color performance for digital cinema projection or other types of digital image projection. Blue wavelengths may be in the range of 430 nm to 480 nm and red wavelengths may be in the range of 600 nm to 680 nm. The pump beam may be in the middle of the green region of visible light which extends from 510 nm to 550 nm. The pump laser may be a neodymium-doped yttrium lithium fluoride (Nd:YLF) laser which emits light at 1047 nm that can be frequency doubled to 523.5 nm, a neodymium-doped yttrium aluminum perovskite (Nd:YAP) laser which emits light at 1079.5 nm that can be doubled to 540 nm, or other lasers with other wavelengths.

FIG. 1 shows an optical system with an optical parametric oscillator that generates two additional colors of light from one pump beam. Pump beam segment 102 passes through first short wave pass (SWP) filter 104 and enters OPO 106 which converts part of pump beam segment 102 into colocated signal, idler, and remaining pump beam segment 108. Colocated signal, idler, and remaining pump beam segment 108 passes to second SWP filter 110 which reflects first idler beam segment 112 and passes colocated signal and remaining pump beam segment 134. First idler beam segment 112 is focused by first lens 114 to form second idler beam segment 116. Second idler beam segment 116 reflects from first minor 118 to form third idler beam segment 120. Third idler beam segment 120 passes into first second-harmonic generator (SHG) 122. First SHG 122 converts part of third idler beam segment 120 to form colocated second harmonic and remaining idler beam segment 124. Colocated second harmonic and remaining idler beam segment 124 passes to third SWP filter 126 which reflects fourth idler beam segment 128 and passes first second-harmonic beam segment 140. Fourth idler beam segment 128 is focused by second lens 130 to form fifth idler beam segment 132. Fifth idler beam segment 132 reflects from first SWP filter 104 to join together with pump beam segment 102.

First SWP filter 104, OPO 106, second SWP filter 110, first lens 114, first minor 118, first SHG 122, third SWP filter 126, and second lens 130 form recirculating optical subsystem 100. Recirculating optical subsystem 100 recirculates the idler beam so it passes multiple times through OPO 106 and first SHG 122. The optical input to recirculating optical subsystem 100 is pump beam 102 and the optical outputs are colocated signal and remaining pump beam segment 134 and first second-harmonic beam segment 140.

After leaving recirculating optical subsystem 100, colocated signal and remaining pump beam segment 134 passes to fourth SWP filter 136 which reflects first signal beam segment 150 and passes remaining pump beam segment 138. First signal beam segment 150 reflects from second mirror 152 to form second signal beam segment 154. Second signal beam segment 154 is focused by third lens 156 to form third signal beam segment 158. Third signal beam segment 158 passes into second SHG 160 which consists of first SHG crystal 162 and second SHG crystal 164. First SHG crystal 162 and second SHG crystal 164 form a walk-off SHG system which converts part of third signal beam 158 to form colocated second harmonic and remaining signal beam 166. Colocated second harmonic and signal beam 166 passes to long wave pass (LWP) filter 168 which reflects second second-harmonic beam segment 172 and passes remaining signal beam 170. Remaining signal beam 170 passes into beam dump 178 and is absorbed in beam dump 178. Second second-harmonic beam segment 172 reflects from third mirror 174 to form third second-harmonic beam segment 176.

Fourth SWP filter 136, second mirror 152, third lens 156, first SHG crystal 162, second SHG crystal 164, LWP filter 168, third mirror 174, and beam dump 178 form beam separation and conversion system 180. The optical input to beam separation and conversion system 180 is colocated signal and remaining pump beam segment 134 and the optical outputs are remaining pump beam segment 138 and third second-harmonic beam segment 176.

FIG. 2 is a schematic view of a recirculating optical subsystem with four relay lenses rather than two relay lenses as shown in FIG. 1. Pump beam segment 202 passes through first SWP filter 204 and enters OPO 206 which converts part of pump beam segment 202 into colocated signal, idler, and remaining pump beam segment 208. Colocated signal, idler, and remaining pump beam segment 208 passes to second SWP filter 210 which reflects first idler beam segment 212 and passes colocated signal and remaining pump beam segment 242. First idler beam segment 212 is focused by first lens 214 to form second idler beam segment 216. Second idler beam segment 216 is focused by second lens 218 to form third idler beam segment 220. Third idler beam segment 220 reflects from minor 222 to form fourth idler beam segment 224. Fourth idler beam segment 224 passes into SHG 226. SHG 226 converts part of fourth idler beam segment 224 to form colocated second harmonic and remaining idler beam segment 228. Colocated second harmonic and remaining idler beam segment 228 passes to third SWP filter 230 which reflects fifth idler beam segment 232 and passes second-harmonic beam segment 244. Fifth idler beam segment 232 is focused by third lens 234 to form sixth idler beam segment 236. Sixth idler beam segment 236 is focused by fourth lens 238 to form seventh idler beam segment 240. Seventh idler beam segment 240 reflects from first SWP filter 204 to join together with pump beam segment 202. First SWP filter 204, OPO 206, second SWP filter 210, first lens 214, second lens 218, minor 222, SHG 226, third SWP filter 230, third lens 234, and fourth lens 238 form recirculating optical subsystem 200.

FIG. 3 shows additional details of the recirculating optical subsystem shown in FIG. 2. In FIG. 3, the beam extents are shown schematically as two lines, whereas in FIG. 1, the beams are shown schematically as single lines. Pump beam segment 202 is focused so that it forms beam waist 250 in OPO 206. First idler beam segment 212 is focused by first lens 214 to collimated second idler beam segment 216 which is collimated. Second idler beam segment 216 is focused by second lens 218 to form third idler beam segment 220 and fourth idler beam segment 224 which forms beam waist 252 in SHG 226. Fifth idler beam segment 232 is focused by third lens 234 to form sixth idler beam segment 236 which is collimated. Sixth idler beam segment 236 is focused by fourth lens 238 to form seventh idler beam segment 240 which joins together with pump beam segment 202 to form beam waist 250 in OPO 206. The beam extent shown in FIG. 3 is exaggerated to more clearly show the difference between focused and collimated.

FIG. 4 shows a non-rectilinear recirculating optical subsystem. Relative to the rectilinear recirculating optical subsystem shown in FIG. 3, the non-rectilinear recirculating optical subsystem shown in FIG. 4 has second SWP 210 moved farther away from OPO 206 and third SWP 230 moved farther away from SWG 226. Also first SWP 204, second SWP 210, mirror 222, and third SWP 230 are rotated approximately 5 degrees clockwise relative to FIG. 2. This produces a non-rectilinear recirculating optical subsystem. The shape of the recirculating optical subsystem is a parallelogram. By changing the positions of first SWP 204, second SWP 210, minor 222, and third SWP 230 and adjusting those four components to the appropriate angles so that the specular reflections circulate the beams around the recirculating optical subsystem, various quadrilateral shapes may be produced.

FIG. 5 shows a method of generating light. In this method, two additional colors of light are generated from one pump beam. Including the remaining pump beam, the total output is three colors of light. In step 500, a pump beam is focused into an OPO. In step 502, the OPO forms a signal beam and an idler beam. In step 504, the idler beam is separated from the signal beam and pump beam. In step 506, the idler beam is focused through at least two lenses into an SHG. In step 508, the SHG forms a second-harmonic beam from the idler beam. In step 510, the second-harmonic beam is separated from the idler beam and the second-harmonic beam is output. In step 512, the idler beam is focused back through at least two lenses to join the pump beam and enter the OPO. In step 514, the remaining pump beam is separated from the signal beam and the remaining pump beam is output. In step 516, the signal beam is focused through a lens into an SHG. In step 518, the SHG forms a second-harmonic beam from the signal beam. In step 520, the second-harmonic beam is separated from the remaining signal beam and the second-harmonic beam is output. In step 522, the remaining signal beam is directed into a beam dump that absorbs the remaining signal beam.

A detailed description of OPOs may be found in U.S. Pat. No. 5,740,190. The wavelengths of the pump, signal, and idler beam are related by the following mathematical expression: 1/λ_(p)=1/λ_(s)+1/λ_(i), where λ_(p) is the wavelength of the pump beam, 1/λ_(s) is the wavelength of the signal beam, and 1/80 _(i) is the wavelength of the idler beam. The wavelengths also depend on various parameters of the crystal such as its size, orientation, and temperature. Some of the requirements for high efficiency conversion include phase matching, good beam quality, and sufficiently high beam density. Q-switched lasers may be used achieve sufficient beam density by using short pulses and low duty cycles. The OPO may be an x-cut lithium triborate (LBO) crystal with noncritical phase matching and temperature controlled at 134.7 degrees Celsius to obtain signal and idler beams at 898 nm and 1252 nm. Alternately, if the temperature is controlled to 135.9 degrees Celsius the signal and idler beam may be at 904 nm and 1242 nm.

A detailed description of SHGs may be found in U.S. Pat. No. 4,019,159, the complete disclosure of which is incorporated herein by reference. SHGs use non-linear optical processes to convert the wavelength of the original light beam into a harmonic wavelength such as half the original wavelength. This is equivalent to doubling the frequency of the light beam. The SHGs shown in FIGS. 1, 2, 3, and 4 may be constructed from LBO.

Phase matching in OPOs and SHGs may be divided into two types. Type I is defined as the condition where two input beams have the same polarization and type II is defined at the condition where two input beams have orthogonal polarization. In the case where there is one input beam, it can be considered two input beams with the same polarization. In FIGS. 1, 2, 3, and 4, OPOs 106 and 206 may be of type I. In FIG. 1, first SHG 122 may be of type II and second SHG 160 may be of type I. In FIGS. 2, 3, and 4, SHG 226 may be of type II.

SWP and LWP filters may be formed by conventional methods such as the deposition of multilayer interference filters with alternating layers of high index and low index of refraction materials that are designed to transmit certain wavelengths while reflecting other wavelengths. The SWP and LWP filters shown in FIGS. 1, 2, 3, and 4 may be vacuum-deposited interference filters on flat, glass substrates.

Optical lenses are used to focus the beams into the nonlinear crystals of the OPOs and SHGs. A narrow focal point (beam waist) is helpful to reach the high power density required for nonlinear optical processes. Additionally, in order for the recirculating optical subsystem to work efficiently, the beam must go around the recirculation path repeating the position of each beam waist within a variation of approximately 10% of the width of the beam waist after 10 circuits. For example, if the beam waist is 80 micrometers wide, the idler beam must go around the recirculating optical subsystem 10 times while drifting less than 8 micrometers.

The recirculating optical subsystem shown in FIGS. 2, 3, and 4 with four lenses has a number of advantages over the recirculating optical subsystem shown in FIG. 1 which has only two lenses. One advantage of the four-lens system is that the beams which travel between the OPO and the SHG may be collimated. This makes the collimated section insensitive to length changes that may result from temperature changes or drift over the lifetime of the system. By placing first SWP filter 204 and second SWP filter 210 close to and in the same optical module as OPO 206, those three components become one optical module which minimizes possible position changes in that module. In the same manner, by placing mirror 222 and third SWP filter 230 close to and in the same optical module as SHG 226, those three components become one optical module. Collimated beams which are insensitive to misalignment are used to cover the relatively long distance between the OPO optical module and the SHG optical module.

A second advantage of the four-lens system is that alignment is much easier because more configurations are possible solutions for optical alignment of the recirculating optical subsystem. Two-lens systems such as the one shown in FIG. 1 generally must be rectilinear to a high accuracy. The four-lens system in FIGS. 2 and 3 has many possible alignment solutions that are not rectilinear such as the parallelogram shape shown in FIG. 4.

A third advantage of the four-lens system is that the issue of keeping the beam waist centered in the crystal can be separated from the issue of keeping the beam waist coming back into the same position each time it travels around the recirculating system. These two issues are closely interrelated during the alignment of the two-lens system.

Other implementations are also within the scope of the following claims. 

What is claimed is:
 1. An optical system comprising: an optical parametric oscillator (OPO); a second-harmonic generator (SHG); a first lens which passes light between the OPO and the SHG; a second lens which passes light between the OPO and the SHG; and a third lens which passes light between the OPO and the SHG.
 2. The system of claim 1 further comprising: a fourth lens which passes light between the OPO and the SHG.
 3. The system of claim 2 wherein the first lens passes a collimated beam segment to the second lens.
 4. The system of claim 3 wherein the third lens passes a collimated beam segment to the fourth lens.
 5. The system of claim 4 wherein the first lens, the second lens, the third lens, and the fourth lens are arranged to form beam segments with a non-rectilinear shape.
 6. The system of claim 4 further comprising: a recirculating optical subsystem; wherein the recirculating optical subsystem focuses a beam waist in the OPO to a same location for 10 circuits around the recirculating optical subsystem with a variation less than 10% of a width of the beam waist.
 7. The system of claim 1 further comprising: a first short wave pass (SWP) filter; a second SWP filter; a mirror; and a third SWP filter; wherein the first SWP filter, the second SWP filter, the minor, and the third SWP filter are arranged to form a recirculating optical subsystem.
 8. The system of claim 7 wherein the first SWP filter and the second SWP filter are mounted in an optical module with the OPO; and wherein the minor and the third optical SWP filter are mounted in an optical module with the SHG.
 9. The system of claim 1 wherein the optical system generates a first infrared beam with a first infrared wavelength and a second infrared beam with a second infrared wavelength.
 10. The system of claim 9 wherein the first infrared beam is frequency doubled to make a first visible beam with a first visible wavelength and the second infrared beam is frequency doubled to make a second visible beam with a second visible wavelength.
 11. The system of claim 10 wherein the first visible wavelength is in the range of 430 nm to 480 nm and the second visible wavelength is in the range of 600 nm to 680 nm.
 12. The system of claim 11 wherein the first visible wavelength is 452 nm and the second visible wavelength 621 nm.
 13. The system of claim 1 wherein the OPO is pumped by a beam with a wavelength in the range of 510 nm to 550 nm.
 14. The system of claim 13 wherein the wavelength is 523.5 nm.
 15. The system of claim 13 wherein the wavelength is 540 nm.
 16. An optical system comprising: a first short wave pass (SWP) filter; an optical parametric oscillator (OPO); a second SWP filter; a first lens; a second lens; a mirror; a second-harmonic generator (SHG); a third SWP filter; a third lens; and a fourth lens; wherein the OPO passes light from the first SWP filter to the second SWP filter, the second SWP filter passes light from the OPO to the first lens, the first lens passes light from the second SWP filter to the second lens, the second lens passes light from the first lens to the mirror, the mirror passes light from the second lens to the SHG, the SHG passes light from the mirror to the third SWP, the third SWP passes light from the SHG to the third lens, the third lens passes light from the third SWP to the fourth lens, the fourth lens passes light from the third lens to the first SWP filter, and the first SWP filter passes light from the fourth lens to the OPO.
 17. A method of generating light comprising: focusing a pump beam into an optical parametric oscillator (OPO); forming an idler beam in the OPO; focusing the idler beam through a first lens and a second lens into a first second-harmonic generator (SHG); and recirculating the idler beam through a third lens and a fourth lens back into the OPO.
 18. The method of claim 17 wherein a beam segment between the first lens and the second lens is collimated; and wherein a beam segment between the third lens and the fourth lens is collimated.
 19. The method of claim 17 further comprising: forming a signal beam in the OPO; separating the idler beam from the signal beam and the pump beam; forming a first second-harmonic beam from the idler beam in the first SHG; and separating and outputting the first second-harmonic beam.
 20. The method of claim 19 further comprising: separating and outputting a remaining pump beam; focusing the signal beam into a second SHG; forming a second second-harmonic beam in the second SHG; separating and outputting the second second-harmonic beam; and dumping a remaining signal beam. 